Destruction of the ozone layer by chlorofluorocarbon 11 (hereafter is called CFC11), which has been used as a blowing agent of thermal insulating material applied to, for example, refrigerators and freezers, is a global issue from the viewpoint of preservation of the global environment.
Based on such background, researchers have been concentrated on the development of thermal insulating materials using novel blowing agents of substitute CFCs or non-CFCs that replace CFC11. A typical example of the substitute CFC blowing agent is hydrochlorofluorocarbon 141b (HCFC 141 b). A typical example of the non-CFC blowing agent is cyclopentane.
These novel blowing agents have higher gas thermal conductivities than that of CFC11, and thereby lower the performance of thermal insulation in refrigerators and the like.
Taking into account the future energy restriction, energy-saving in refrigerators and the like is an inevitable issue. Improvement in performance of thermal insulation is one of the possible solutions.
As discussed above, the conventional thermal insulating materials have conflicting issues, that is, the actual drop in performance of thermal insulation due to the use of substitute CFCs and the requirement for improvement in performance of thermal insulation to attain energy-saving of apparatuses using thermal insulating materials.
Vacuum thermal insulating materials have been proposed as potential means for solving such conflicting issues as disclosed in, for example, the gazettes of the Japanese unexamined patent application (TOKKAI) Nos. Sho-57-173689 and Sho-61-144492. These vacuum thermal insulating materials are prepared from inorganic powder. These vacuum thermal insulating materials were obtained by filling a film-like plastic vessel with silica powder and sealing the vessel under reduced pressure.
A major advantage of the conventional vacuum thermal insulating materials is that they can be manufactured at the degree of vacuum of 0.1 to 1 mmHg which can be readily performed in the industrial process. The conventional vacuum thermal insulating materials containing fine silica powder have improved performance of thermal insulation at the same degree of vacuum, compared with those without silica powder.
The principle of thermal insulation in the vacuum thermal insulating material is to remove a gas that transmits heat, for example, the air, from essential parts of a thin case to be thermally insulated, such as doors and walls. It is, however, difficult to produce a high vacuum in the industrial (mass production) level. The practical degree of vacuum ranges from 0.1 to 10 mmHg. The vacuum thermal insulating materials accordingly have to obtain the desired performance of thermal insulation at the degree of vacuum of this range.
In the process of heat conduction via the air, the physical property affecting the performance of thermal insulation is the mean free path of gas molecules.
The mean free path represents a distance by which one molecule consisting of a gas, for example, the air, travels before colliding with another molecule. When voids formed in the air are greater than the mean free path, molecules collide with each other in the voids to produce heat conduction via the air. Such vacuum thermal insulating material has an increased thermal conductivity. When the voids are smaller than the mean free path, on the contrary, the vacuum thermal insulating material has a small thermal conductivity. This is because there is substantially no heat conduction due to the collision of gas molecules, for example, the air molecules.
In order to enhance the performance of thermal insulation in the vacuum thermal insulating material, it is required to form voids that are smaller than the mean free path of molecules. Filling fine powder having small particle diameters, such as silica powder, makes small voids and substantially eliminates the heat conduction due to the collision of the air molecules. As a result, the performance of thermal insulation is improved in the vacuum thermal insulating material.
The conventional structure using a large mass of silica powder, however, undesirably increases the weight of the vacuum thermal insulating material and raises the manufacturing cost.